The present invention relates to an improved process for the production of thiocarbohydrazide.
Several processes for the manufacture of thiocarbohydrazide (TCH) are known. TCH may be obtained in moderate yields during the hydrazinolysis of thiophosgene in an ether or water reaction medium. It is also known to manufacture TCH by hydrazinolysis of diethylxanthate and by heating the two reaction components in the absence of a solvent. Yields of 70-74% of theory may be obtained by the latter process.
It is also known to prepare thiocarbohydrazide through conversion of dialkyltrithiocarbonates with hydrazine. Cyclic trithiocarbonate may also be used for this synthesis.
The most common synthesis of thiocarbohydrazide is, however, the conversion of carbon disulfide with hydrazine. Hydrazinium-dithiocarbazinate forms according to equation (1): EQU CS.sub.2 +2H.sub.2 NNH.sub.2 .fwdarw.H.sub.2 NNHCSSH.NH.sub.2 NH.sub.2 ( 1)
This compound is then converted to thiocarbohydrazide with evolution of hydrogen sulfide according to equation (2): EQU H.sub.2 NNHCSSH.NH.sub.2 NH.sub.2 .fwdarw.H.sub.2 NNHCSNHNH.sub.2 +H.sub.2 S (2).
Better yields and pure product may be obtained when the hot aqueous solution of the hydrazinium-dithiocarbazinate is digested with lead oxide (Stolle, et al., Ber. 41, 1099 (1908)). The yields of thiocarbohydrazide can also be increased when conducting the decomposition of hydrazinium-dithiocarbazinate in aqueous solution in the presence of hydrazine (U.S. Patent No. 2,726,263). However, increasing dilution of the hydrazine-containing reaction medium with water decreases the yield of TCH. Further, use of a water-free solvent for hydrazine (e.g., methyl, ethyl, or propyl alcohol) does not increase the TCH yield.
In each of the above-described processes, any excess hydrazine must be recovered. Such recovery is costly, time consuming and troublesome due to the reactivity and instability of hydrazine.
U.S. Pat. No. 4,294,985 teaches that the mother liquor containing the excess hydrazine may be recycled instead of being recovered. However, recycling of the hydrazine-containing liquor creates new problems. More specifically, recycling results in the build up of impurities which can reduce the yield of TCH. Further, some of the hydrazine containing liquor must be purged from the system after each recycle resulting in loss of hydrazine.
It is also known to convert hydrazinium dithiocarbazinate to TCH thermally. Yields of approximately 70% may be obtained by such thermal conversion processes.
Production of TCH by pyrolyzing a reaction mixture containing carbon disulfide and hydrazine hydrate is also known. The yields of such processes are detrimentally affected by side reactions which result in the formation of ammonia and sulfur. These side reactions are further catalyzed by the sulfur formed.
It would therefore be advantageous to develop a process for producing TCH (1) which did not require removal or recycling of excess of hydrazine, (2) which suppressed side reactions resulting in the formation of ammonia and sulfur and (3) which produced TCH in high yield.